Field of the Invention
The present invention relates to a method for monitoring the effectiveness of a heated catalytic converter and, more particularly, of such a converter placed upstream of a three-way catalytic converter in an outlet line for the exhaust gases of an internal combustion engine.
A device for treating the exhaust gases of an internal combustion engine is known which comprises, as represented in FIG. 1 of the appended drawing, on the one hand, two catalytic converters 1 and 2 arranged in the exhaust line 3 of the engine and, on the other hand, means 4 for injecting air into this line, upstream of the catalytic converter 1. In this respect, reference may be made to the document No. 910840 entitled "New Potential Exhaust Gas After-treatment Technologies for Clean Car Legislation" by I. Gottberg and associates, published by the Society of Automotive Engineers in the United States of America.
As described in this document, the catalytic converter 1 is a converter which is heated, when the engine is being started from cold, by a source of electrical energy 5 such as a battery connected to the converter 1 through a power module 6 controlled by a computer 7. The heated converter 1 comprises a metal support covered by a coating of precious metal such as platinum, capable of catalyzing the oxidation of unburnt hydrocarbons and or carbon monoxide in order to convert them into less harmful chemical species. Passing an electrical current through this support thus covered causes Joule-effect heating of the converter capable of rapidly bringing the exhaust gases passing through it up to a temperature allowing these oxidation reactions. The injection 4 of air into these exhaust gases provides the additional oxygen necessary for these reactions. The oxidation reactions thus initiated are exothermal and contribute to heating up the catalytic converter 2 placed downstream of the heated catalytic converter 1, so that it can, after the period in which the engine is started from cold, take over most of the treatment of all
harmful species contained in the exhaust gases of the engine, that is to say, apart from the unburnt hydrocarbons and carbon monoxide which have already been mentioned, the oxides of nitrogen which have to be reduced into less harmful species. The catalytic converter 2 is therefore called "three-way" owing to its triple action on the unburnt hydrocarbons, the carbon monoxide, and said oxides of nitrogen.
Of course, such means for treating the exhaust gases are effective only if the catalytic converters used are in good working order. Now, the coatings of precious metals used to oxidize some exhaust gases (platinum for example), for reducing others (rhodium for example) or for storing oxygen (cerium) degrade over time under the effect of "poisons" or other attacks such as those resulting from the combustion of unburnt hydrocarbons inside the converter. The active surface of the metal coating therefore decreases, which lowers the overall effectiveness of the converter.
Increasingly strict antipollution standards now envisage motor vehicles being equipped with on-board means for permanently monitoring the effectiveness of the catalytic converters used, so that a degradation of this effectiveness can be detected and remedied before this effectiveness falls below a minimum permissible level.
To this end, French Patent Application No. 91 13237 filed on 28 Oct. 1991 by the Applicant Company corresponding to copending U.S. application Ser. No. 08/232,228, describes a method for monitoring the effectiveness of a catalytic converter of the "three-way" type, like the catalytic converter 2 of the device of FIG. 1, this method being based on evaluating the converter's capability to store oxygen.
Such a method is applicable when the catalytic converter in question has reached its working temperature. However, in the case where at least one catalytic converter is heated, it is appropriate to monitor its effectiveness as early as possible, that is to say as was seen above, to monitor its capability to:
1) oxidize the unburnt hydrocarbons and the carbon monoxide,
2) rapidly bring the exhaust gases and the three-way catalytic converter placed downstreamup to a minimum working temperature of the order of 350.degree. to 400.degree. C., so as to reduce the production of harmful species to a minimum.
The object of the present invention is therefore to provide a method for monitoring the effectiveness of a heated catalytic converter placed in the outlet line of the exhaust gases of an internal combustion engine.